Two-stage clamping mechanism

ABSTRACT

A hydraulically actuated two-stage clamping mechanism for moving a movable member into and out of cooperative relationship with a stationary member. A pair of rigid link arms is pivotally interconnected at one end of each of the arms to form a toggle which comprises the first stage of the mechanism. The free end of one of the toggle arms is pivotally connected to a stationary portion while the free end of the other arm is pivotally connected to a movable member. The pivotally interconnected ends of the toggle arms define a knee-type joint which incorporates an integral hydraulic rotary actuator which upon the application of hydraulic pressure, causes relative motion between the arms and thereby moves the movable member toward and away from the stationary member. The second stage of the mechanism is a large diameter, short travel clamping piston to firmly hold the movable and stationary members together. Additionally, adjustment means are provided to vary the initial spacing between the two stationary members.

United States Patent [72] Inventor David I. McDonald Cincinnati. Ohio[2|] Appl No 21,446 [22] Filed Mar. 20. I970 [45] Patented Aug. 10, 1971I73] Assignee Cincinnati Milltron lnc.

Cincinnati, Ohio [54] TWO-STAGE CLAMPING MECHANISM I0 Claims, 7 DrawingFigs. [52] [1.5. CI U l8/30 LV [5| Int. Cl r v v r r. B29! 1/00 [50]Field of Search 18/30 L I 56] References Cited UNITED STATES PATENTS3,345,69l l0/l967 Aoki I8/30 LV 3.4 I 8,692 l 2/1968 Valyi .r 18/30 LVPrimary Examiner-J. Howard Flint, Jr. Attorneys-Howard T. Keiser andAlfred J. Mangels A hydraulically actuated two-stage clamping rigid linkarms is pivotally interconnected at one end of each of the arms to forma toggle which comprises the first stage of Patented Aug. 10, 19713,597,798

5 Sheets-Sheet 1 mvsw'roa DAWD I. McDONN..D

F Qf y -W- ATTORNEYS Patented Aug. 10, 1971 3,597,798

5 Sheets-Sheet 2 Patented Aug. 10, 1971 5 Sheets-Sheet 5I'IIIIIIIII'IIJI TWO-STAGE CLAMPING MECHANISM BACKGROUN D OF THE INVENTION This invention relates to hydraulically operated clampingdevices and more particularly to a two-stage, hydraulically operatedclamp device incorporating a rotary hydraulic actuator at the knee of atoggle to provide rapid opening and closing of a mold and alsoincorporating a large-diameter short travel clamping piston for firmlyholding the mold sections together.

In clamping devices associated with molding machines such as, forexample, diecasting machines and plastics molding machines of varioustypes, a split mold or die is frequently util ized to define the shapeof the article to be formed and, therefore, it is necessary to open andclose the die or mold at periodic intervals to permit removal of thecast or molded article. The opening and closing operation is desirablyperformed rapidly so as to permit the manufacture of a large number ofparts within a given time period. In order to minimize the timenecessary to complete a given opening and closing cycle, it is necessarythat the closing and opening velocities of the mold be high or that thedistance between the molds be small. How ever, for a given mold theseparation distance between the mold portions is generally fixed andthus the closing and opening velocities are the only remainingvariables. It is also desirable that the final velocity of the movingmold portion with respect to the stationary mold portion be small sothat when contact occurs the impact force which results is minimal andthus does not cause damage to the mold.

A desirable mechanism for imparting the desired motion to the movingmold member is a so-called toggle-type link wherein a pair of linkmembers is pivotally interconnected at one end thereof and has itsopposite ends pivotally connected to a stationary member and to themoving mold member, respectively. Such a mechanism results in asubstantially sinusoidal velocity profile in that when the link membersare overlapped the velocity imparted to the moving element is very high,while after the toggle is opened substantially such that the includedangle between the link members is large, the closing velocity tends todecrease up to the point where the toggle is completely extendedwhereupon the velocity is zero. Preferably, the molds are in firmcontact at the point where the toggle is completely extended in order toprevent flashing of the molded material from the mold cavity.

To actuate the abovc-described toggle-type linkage systems, it has beenthe practice to employ a linear hydraulic cylinder having its axissubstantially perpendicular to the path of travel of the moving moldmember so that a force is applied at the knee of the toggle tending toforce the toggle arms apart and thus impart motion to the movable moldmember. That approach, however, requires a relatively high capacityhydraulic cylinder because initially the principal component of theforce is acting in a direction which is at right angles to the desiredpath of travel of the movable mold. Additionally, because of theperpendicular orientation of the cylinder, 3 large transverse load isapplied to the various interconnections of the link system, which causesthe movable platen to bear against the tie rods upon which it isslidably carried and thus tends to cause rapid wear on the bearingsurfaces of the platen which bear against the tie rods. In addition, theside load tends to cause deflection in the tie rods, which is highlyundesirable, and thus requires that stronger tie rods be utilized inorder to resist the transverse deflection imposed by the perpendicularlyapplied forces. Furthermore, when rapidly operating mold-closing systemsit is desirable that final contact between the molds be made lightly topreclude possible damage thereto caused by impact forces. However,slowing down the moldclosing system to prevent impact also increases thecycle time, which is undesirable.

It is an object ofthe present invention to obviate the abovedcscribeddifficulties.

SUMMARY OF THE INVENTION Briefly stated, in accordance with one aspectof the present invention, a two-stage clamping mechanism is providedbetween a stationary member and a movable member spaced therefrom. Themechanism comprises a pair of link members pivotally interconnected atone end thereof to form a knee joint and having their opposite endspivotally connected to said stationary member and said movable member,respectively. Oppositely directed torques are applied to the end of eachof the lines at the knee joint to thereby impart linear motion to themovable member and also to avoid the imposition of ex cessively largeloads perpendicular to the direction of travel of the movable member.The oppositely directed torques are provided by a rotary hydraulicactuator positioned at the knee joint.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a fragmentary side view,partially in section, showing the clamping mechanism of the presentinvention as applied to an injection molding machine to impart motion tothe movable platen thereof FIG. 2 is a fragmentary elevational view,partially in section, showing the clamp mechanism of FIG. I in its fullyextended position.

FIG. 3 is a fragmentary top view of the device shown in FIG. 2.

FIG. 4 is a longitudinal cross-sectional view through the link armsshowing the structural arrangement thereof at the point at which thearms are interconnected.

FIG. 5 is a fragmentary crosssectional view taken along the line 55 ofFIG. 4.

FIG. 6 is an end view of the injection molding machine of FIG. 1, takenalong the line 6-6 thereof.

FIG. 7 is a fragmentary cross-sectional view taken along the line 7-7 ofFIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand particularly to FIG. I thereof, there is shown the clamp and moldsection of an injcc tion molding machine. The machine includes a base [0on which is positioned a stationary support II and a stationary platenI2 spaced from stationary support II and to which is attached astationary mold member [3. A plurality of tie rods I4 are attached toboth stationary support II and stationary platen l2 and extendtherebetween in substantially parallel, spaced relationship. A movableplaten I5 is slidably positioned on tie rods 14 between stationarysupport 11 and stationary platen I2 and carries a cooperating movablemold member 16 which is adapted to engage with stationary mold member 13to define a mold cavity 17 (see FIG. 2). An injection nozzle 18 isprovided to conduct the material to be molded into mold cavity 17. Theconstruction of injection nozzle 18 and the injection system of which itis a part are well-known to those skilled in the art and, therefore,will not he further described herein.

Movable platen 15 can comprise a pair of spaced plates 19, 20 havingtheir faces in parallel relationship and being slidably positioned ontie rods I4. Plates I9 and 20 can include bearing surfaces 21 whichcontact tie rods 14 and reduce friction and wear therebetween. A spacer22 of desired axial extent can be interposed between plates I9 and 20 tocompensate for the thickness of the various molds which can be securedto fixed platen I2 and movable platen I5. Additionally, a hydraulicallyoperated knockout system can be employed to dislodge the molded piecefrom movable mold member 16. As shown, the knockout system can compriseone or more hydraulic cylindets 23 utilized to move a pluralityofejector pins (not shown) which are positioned within movable moldmember I6 to eject the completed part therefrom upon the conclusion ofthe molding cycle. Such ejection systems are well-known to those skilledin the art, and, therefore, will not be described in detail herein.

The link mechanism of the present invention is shown in FIGS. I, 2, and3 and comprises two pairs of link arms 24, 25 interconnected at one endthereof to provide a toggle-type arrangement. The free end of one pair24 of link arms is pivotally connected to stationary support I I bymeans of a pin 26 which is carried by a clevis 27 attached to thestationary support. Similarly, the other pair of link arms 25 ispivotally connected to the rcarmost portion 19 of movable platen I5 bymeans of a pin 28, which is carried by a clevis 29 secured to therearmost portion l9 of movable platen 15. Each of the pins 26, 28 can besecured against endwise motion by means of end caps 30 bolted thereto asshown more clearly in FIG. 3. Alternatively, other retaining means couldalso be used, such as, for example, retaining rings (not shown) or aninterference fit between the pins and the respective clevises, or thelike. In any event, it is only necessary that link arms 24, 25 bepivotable about the pins 26, 28 and whether or not pins 26, 28 aremovable within clevises 27, 29, respectively, is a matter of choice.

The clamp system of the present invention is shown in FIG. I with themold members 13 and 16 in the open position and the toggle retracted toa point in its path of travel such that the resulting included anglebetween link anns 24 and 25 is small. To close the mold and bring linkarms 24 and 25 to the position shown in FIG. 2 it is necessary that aforce be applied to the toggle forcing link arms 24 and 25 apart,whereupon the motion of the arms causes movable platen to move linearlyalong tie rods I4 into cooperative relationship with stationary platen12 so that the mold portions are in contact and thus define mold cavity17.

When the mold is closed and the toggle is in the fully extended positionas shown in FIGS. 2 and 3, the material to be molded may be injectedinto the mold cavity through an aperture in stationary mold member I3 toform the article desired. During the injection phase of the cycle, thematerial to be molded must of necessity be injected under high pressuresince it frequently is a viscous material such as, for example, a moltenpolymer, and the high pressure is necessary to cause the same to flow.However, the high pressure necessary to inject the material also resultsin forces within mold cavity 17 tending to separate mold members 13 and16, and thus it is necessary that link arms 24 and 25 be madesufficiently strong so that they will resist the axial load impartedthereto as a result of the pressure of the injected molding material andpreclude separation of mold members 13 and 16. When mold cavity [7 hasbeen filled and the molded article has solidified, the toggle isretracted and is again brought to the position shown in FIG. I, in whichmovable platen I5 is spaced from stationary platen I2 and the completedpart can then be ejected from the mold cavity by means of the ejectionsystem shown generally in FIGS. I and 2.

The link toggle system of the present invention incorporates anintegral, rotary hydraulic actuator 3| at the knee of the toggle or thepoint at which link arms 24 and 25 are interconnected. Actuator 31 isshown in cross section in FIGS. 4 and 5.

As shown, link arms 24, which are pivotally connected to clevis 27secured to stationary member ll, are spaced transversely and betweenthem is positioned an annular closure ring 32, which is securely heldbetween the link arms 24 as by means of, for example, bolts 33. Each ofthe link arms includes an aperture 34 (see FIG. 5) which is coaxial withannular ring 32. Positioned within those apertures is a pin 35 whichoutwardly beyond the outermost faces of link arms 24 and also passesthrough corresponding apertures 36 in link arms 25 to provide thepivoted connection therebetween.

Pin 35 is of generally cylindrical shape, with an outer diametercorresponding to the diameter of apertures 34 and 36, and includes acoaxial, stepped portion 37 of greater diameter positioned substantiallycentrally along the axis of pin 35. Stepped portion 37 extends axiallyalong pin 35 a distance corresponding to the spacing between theinnermost surfaces of link arms 24 and extends radially beyond apertures34 lo define a substantially fluid tight annular channel bounded by theoutermost surface of stepped portion 37, the innermost surface ofannular ring 32, and the innermost surfaces of link arms 24. Pin 35 hasan end cap 38 bolted to each end thereof, which end cap is also securedto link arms 25, as by means of bolts 39. Thus, both the outermost linkarms 25 and the innermost link arms 24 are pivotally interconnected bymeans of pin 35.

Extending from the outermost surface of stepped portion 37 of pin 35 andinto the annular channel formed between pins 35 and ring 32 are a pairof oppositely disposed vane members 40, which are retained in slots 41in pin 35 as by means of dowel pins 42. Each vane 40 includes atransverse slot 43 at its outer end to receive a sealing strip 44, whichis spring biased by springs 45 into contacting relationship with theinner sur face of annular ring 32.

Also extending within the annular chamber defined by the outermostsurface of stepped portion 37 of pin 35 and annular ring 32 is a pair ofoppositely disposed abutments 46, which are secured to annular ring 32as by means of bolts 47 and are secured against rotation within thechannel by key members 48. As was the case with vanes 40, abutments 46also include transverse slots within which sealing strips 49 arepositioned and spring biased by means of springs 50 to bear against theoutermost surface of stepped portion 37 of pin 35. Additionally,abutments 46 include inwardly directed chamfers 51 positionedtransversely and located between the sides and in nermost surface ofabutments 46. The purpose of chamfers 5] will be explained hereinafter.It can thus be seen that abutments 46 divide the annular chamber into apair of substantially equal, arcuate chambers 52, 53, which are swept byvanes 40 when pin 35 caused to rotate with respect to annular ring 32.

Pivot pin 35 includes a pair of bores 54, 55, coaxial therewith whichextend nartially within pin 35 without meeting. Extending tangentiallyfrom the innermost terminus of each of the axial bores 54, 55 are a pairofoppositely disposed, parallel channels 56, 57, respectively, incommunication with chambers 6t, 62, which, as shown in FIG. 4, are ofsubstantially triangular cross section and are defined by chamfers SI ofabutments 46, vanes 40, and the outermost surface of stepped portion 37of pivot pin 35. A tangential bore 58, 58a is provided in each ofendcaps 38 and terminates at the center thereof to cooperate with an axialbore 59, 59a, respectively, which is in aligned relationship with axialbores 54 and 55, respectively, in pivot pin 35. Sealing rings 60 areprovided to prevent leakage of hydraulic fluid along pin 35.

It can thus be seen that communication is established between chamber 6|and one side of the toggle assembly through channels 56, and bores 54,59, and 58. When hydraulic pressure is applied through bores 58, 59, and54 to the left side of the assembly as shown in FIG. 5, the pressure istransmitted through channels 56 to both chambers 61 and 62. At the sametime, the right side of the assembly as shown in FIG. 5, bores 58a, 59a,and 55, and channels 57 are vented to a hydraulic reservoir through asuitable hydraulic circuit (not shown). The pressure in chambers 61 and62, since it is greater than that in arcuate chambers 52 and 53, actsagainst vanes 40 and thus causes pivot pin 35 to rotate with respect toannular ring 32. The torque of rotation is dependent upon the vane areaand the pressure differential which exists across the vane. Rotationwill continue until either the pressure differential is reduced to zeroor the vanes have swept an arc of slightly less than I and havecontacted the opposite abutments 46. Similarly, rotation in the reversedirection can be accomplished by interchanging the hydraulic connectionsby means of a suitable valve (not shown) such that bores 58a, 59a, and55 are pressurized and bores 58, 59, and 54 are vented to the hydraulicreservoir (not shown).

It can be seen that the application of hydraulic pressure to one oftangential bores 58 or 58a will cause the toggle arms to move withrelation to each other in one direction while imparting pressure to theother tangential borc will cause the toggle arms to move with respect toeach other in opposite direction. It is thus apparent that the kneejoint of the toggle comprises an integral rotary hydraulic actuator toimpart a torque to each of the toggle arms and thus cause the same toopen or close, thereby also causing the movable platen to move forwardor back and, in turn, causing the mold members to either move toward oraway from each other.

Frequently, the injection pressure of the material injected into themold cavity in injection molding machines is very large, for example, onthe order of about 20,000 psi. Thus, when molds are employed having alarge projected area in a plane parallel to the major surfaces of themovable platen, a high clamping force is required in order to assurethat the molds do not separate because of the high axial forces imposedby the injected material, and thereby permit the material to flash fromthe mold cavity and form a thin, finlike projection of material on thefinished part along the mold parting line. The additional clamping forcerequired to prevent a flashing in those instances can be provided by anadditional or auxiliary clamping piston 65 (see FIG. 1) which serves tokeep mold members l3, 16 in firm contact when the mold is closed.Clamping piston 65 has a large force area 66 and is intended to be movedonly a small distance so that only a small quantity of hydraulic fluidis required to actuate it. Additionally, clamping piston 65 ispreferably of the double'acting type so that it can be retracted uponcompletion of the molding cycle. The auxiliary clamping piston shown incross section in FIG. 1 comprises a large area double-acting piston 65which is slideably positioned in stationary support I]. The latterincorporates suitable hydraulic fluid passages 67, 68 to alternatelypressurize front face 66 and rear face 69 of clamping piston 65 andthereby cause the latter to move to the right as shown in FIG. 1, or tothe left, as desired.

An additional advantage which results from the utilization of auxiliaryclamping piston 65 in conjunction with the toggle mechanism of thepresent invention is that the toggle mechanism can be operated veryrapidly and caused to stop movable mold member [6 just short ofstationary mold member 13 without any contact taking place between thetwo, if desired. Since there is then no contact between the moldmembers, there would be no impact forces and thus the initial closingmovement can be made very rapidly by means of the toggle. Final movementof movable mold member 16 into contact with stationary mold member I3 isaccomplished by imposing hydraulic pressure on front face 66 ofauxiliary clamping piston 65 through passage 67. This operation movesmovable mold member 16 the remaining distance to bring it into contactwith stationary mold member l3 and then firmly holds the two moldmembers in cooperative engagement while the mold material is injectedinto mold cavity [7 formed therebetween. When the molded article hassolidified to the required extent, auxiliary clamping piston 65 can bemoved toward the left as shown in FIG. I by applying hydraulic pressureto rear face 69 of clamping piston 65 through passage 68, thusseparating the mold members slightly. At that point, the togglemechanism can be actuated to rapidly move movable mold member 16 awayfrom stationary mold member 13. If desired, both of those movements canbe made simultaneously to further reduce the cycle time and thusincrease the operating efficiency of the clamping mechanism of thepresent invention.

in order to compensate for differences in size between molds and topermit the clamping mechanism of the present invention to be set up forthe most rapid and most efficient mode of operation, adjusting means areprovided whereby stationary support I] of the clamping system can bemoved closer to or further from stationary platen 12. To permit movementto take place, stationary support H is slidably mounted on ways 70 asshown in FIG. 1, tie rods l4, along which movable platen travels, extendbeyond rear face 71 of stationary support ll and have external threads72 formed thereonv Adjusting nuts 73 are provided on each of tie rods 14and include an enlarged flange 74 at the end closest to rear face 7! ofstationary support ll. Flange 74 on nuts 73 are engageable with annularretainers 75 attached to rear face 7| of stationary support I] incoaxial relationship with the several tie rods 14. Thus, when adjustingnuts 73 are turned, stationary support II is caused to move toward oraway from stationary platen 12.

As shown in FIGS. I and 6, each of adjusting nuts 73 carries a sprocketmember 76 about which an endless chain 77 passes to interconnect all ofsprockets 76 and drive them simultaneously. A driving sprocket 78 isrotatably carried on a bracket 79 adjustably positioned on rear face 7|of stationary support 11. A large bevel gear 80 is keyed to a shaft 8|which also carries driving sprocket 78 and meshes with a perpendicularlypositioned driving pinion 82, which is keyed to a shaft 83 to which ahandwheel 84 can be connected. Thus, turning handwheel 84 in onedirection causes driving sprocket 78 to rotate and simultaneously drivesall of sprockets 76 and their associated adjusting nuts 73 in onedirection, thus moving stationary support I], while reversing thedirection of rotation of handwheel 84 causes adjusting nuts 73 to rotatein the opposite direction, thereby moving stationary support ll in theopposite direction. Since adjusting nuts 73 are rotatably engaged withannular retainers 75 on rear face 7[ of stationary support 11, bothforward and rearward motion can be im parted to stationary support 1].To adjust the tension in chain 77, bracket 79 can be moved by means ofshaft 85, which is threadably carried in block 86. The latter alsothreadably carries shaft 83 by means of which the position of bracket 79is adjusted. When stationary support 11 have been set at the desiredposition, the adjusting mechanism can be locked by rotating a shaft 87which is also threadably carried by block 86. Shaft 87 bears against awedge member 88 which, in turn, is forced against shaft 83 of theadjusting mechanism, thereby preventing rotation of shaft 83 and lockingit in the desired position. If desired, a cover 89 can be provided toenclose the adjusting mechanism and protect it from dirt and inadvertentexternal damage.

It can thus be seen that the present invention provides an improvedclamp system for injection molding machines and the like by providing afast operating, positive action, high clamping force, adjustabletwo-stage clamping mechanism in corporating a rapid action toggle forinitial closure of the mold members and a large diameter, short travelclamping piston for final closure of the mold members and for clampingthe same. By applying the motive force which actuates the toggle in theform of a torque applied to each of the lever arms thereof, greateraxial forces and thus greater speed is applied to the movable platenduring the initial portion of the clamping cycle when high speed isdesired. Additionally, the hydraulic toggle of the present inventiondoes not impart high side loads on the platen bearings since it does notapply large forces in a direction perpendicular to the direction ofmotion of the movable platen, and thus results in lower wear of theplaten bearings. Furthermore, it can be seen that the improved clampingdevice of the present invention provides an extremely compact mechanismsince the actuator and linkage for the toggle are combined in a unitarystructure.

Although described in terms of an injection molding machine, it would bereadily understood by those skilled in the art that the inventionhereinabove described can also be utilized in diecasting machines, blowmolding machines, or in any other apparatus wherein one member is movedrelative to another and that movement is desirably a rapid one to reducethe time between an extreme open position and an extreme closedposition.

While particular embodiments of the invention have been illustrated anddescribed, it will be apparent to those skilled in the art that variouschanges and modifications can be made without departing from the spiritand scope of the invention, and it is intended to cover in the appendedclaims all such changes and modifications that are within the scope ofthis invention.

lclaim:

I. in a mold-clamping mechanism for moving a movable member carrying onepart of a mold into engagement with a first stationary member carrying asecond part of said mold, said movable member having a front and rearside and being connected through a link system to a second stationarymember positioned on a base and facing the rear side of said movablemember, said moving and stationary members having their principal facessubstantially parallel, and said moving member being supported forlinear movement toward and away from said first cooperating stationarymember, the improvement comprising:

a. a first link member having a first end and a second end spaced fromsaid first end, said first end being pivotally attached to the rear sideof said movable member;

b. a second link member having a first end and second end spaced fromsaid first end, said first end being pivotally attached to said secondstationary member;

c. said second end of said first link member being pivotally attached tosaid second end of said second link member to form a knee joint;

d. a rotary hydraulic actuator positioned at said knee joint forapplying oppositely directed torques to the ends ol each of said linksat said knee joint to thereby change the included angle between saidlinks and thus move said movable member relative to said stationarymembers; and

e. means for increasing the contact pressure between said mold parts topreclude separation thereof during the molding process.

2. The clamping mechanism of claim I wherein said rotary hydraulicactuator is integral with said first and said second links and definessaid knee joint.

3. The clamping mechanism of claim 2 wherein the second end of one ofsaid links is attached to a cylindrical pivot which carries a pair ofoutwardly directed, substantially radially extending vanes and thesecond end of the other of said links is attached to a ring memberconcentric with said pivot, said ring member having an inner diametergreater than the outer diameter of said pivot, said vanes contacting theinner surface of said ring member, a pair of stops diametricallypositioned on the inner surface of said ring member and extendinginwardly to said pivot to define a pair of arcuate chambers thercbetweenand limit the relative rotation between said first and second links, andmeans for selectively introducing hydraulic fluid under pressure toeither side of said vanes to cause said vanes to move relative to saidstops and thereby im part relative rotation between said links aboutsaid knee joint.

4. The clamping mechanism of claim 3 wherein said hydraulic fluid isintroduced through bores provided in said pivot.

5. The clamping mechanism of claim 4 wherein said vanes and said stopsinclude spring-biased sealing means to prevent leakage of hydraulicfluid around the ends thereof.

6. The clamping mechanism of claim 2 wherein said pressure-increasingmeans comprises a hydraulic piston slidable within said secondstationary member and having said first end of said second link memberpivotally attached thereto.

7. The clamping mechanism of claim 6 including means for varying thespacing of said second stationary member from said first stationarymember.

8. The clamping mechanism of claim 7 wherein said position varying meanscomprises:

a. ways carried by said base upon which said second sta' tionary membercan move toward and away from said first stationary member;

b. tie rods extending between said first and said second stationarymembers, said tie rods including external screw threads formed thereinin the vicinity of said second stationary member;

. adjusting means positioned for movement along said screw threads; d.means carried by said second stationary member cooperable with saidadjusting means; and e. means for moving said adjusting means along saidscrew threads, whereby to cause said secondary member to move toward oraway from said first stationary member. 9. The clamping mechanismot'claim 8 wherein said moving means comprises an endless chain whichpasses around sprocket members carried by said adjusting means to moveeach of said adjusting means an equal distance, and means for movingsaid endless chain.

[0. The clamping mechanism ofclaim 9 including means for locking saidmoving means in any desired position.

1. In a mold-clamping mechanism for moving a movable member carrying onepart of a mold into engagement with a first stationary member carrying asecond part of said mold, said movable member having a front and rearside and being connected through a link system to a second stationarymember positioned on a base and facing the rear side of said movablemember, said moving and stationary members having their principal facessubstantially parallel, and said moving member being supported forlinear movement toward and away from said first cooperating stationarymember, the improvement comprising: a. a first link member having afirst end and a second end spaced from said first end, said first endbeing pivotally attached to the rear side of said movable member; b. asecond link member having a first end and second end spaced from saidfirst end, said first end being pivotally attached to said secondstationary member; c. said second end of said first link member beingpivotally attached to said second end of said second link member to forma knee joint; d. a rotary hydraulic actuator positioned at said kneejoint for applying oppositely directed torques to the ends of each ofsaid links at said knee joint to thereby change the included anglebetween said links and thus move said movable member relative to saidstationary members; and e. means for increasing the contact pressurebetween said mold parts to preclude separation thereof during themolding process.
 2. The clamping mechanism of claim 1 wherein saidrotary hydraulic actuator is integral with said first and said secondlinks and defines said knee joint.
 3. The clamping mechanism of claim 2wherein the second end of one of said links is attached to a cylindricalpivot which carries a pair of outwardly directed, substantially radiallyextending vanes and the second end of the other of said links isattached to a ring member concentric with said pivot, said ring memberhaving an inner diameter greater than the outer diameter of said pivot,said vanes contacting the inner surface of said ring member, a pair ofstops diametrically positioned on the inner surface of said ring memberand extending inwardly to said pivot to define a pair of arcuatechambers therebetween and limit the relative rotation between said firstand second links, and means for selectively introducing hydraulic fluidunder pressure to either side of said vanes to cause said vanes to moverelative to said stops and thereby impart relative rotation between saidlinks about said knee joint.
 4. The clamping mechanism of claim 3wherein said hydraulic fluid is introduced through bores provided insaid pivot.
 5. The clamping mechanism of claim 4 wherein said vanes andsaid stops include spring-biased sealing means to prevent leakage ofhydraulic fluid around the ends thereof.
 6. The clamping mechanism ofclaim 2 wherein said pressure-increasing means comprises a hydraulicpiston slidable within said second stationary member and having saidfirst end of said second link member pivotally attached thereto.
 7. Theclamping mechanism of claim 6 including means for varying the spacing ofsaid second stationary member from said first stationary member.
 8. Theclamping mechanism of claim 7 wherein said position varying meanscomprises: a. ways carried by said base upon which said secondstationary member can move toward and away from said first stationarymember; b. tie rods extending between said first and said secondstationary members, said tie rods including external screw threadsformed therein in the vicinity of said second stationary member; c.adjusting means positioned for movement along said screw threads; d.means carried by said second stationary member cooperable with saidadjusting means; and e. means for moving said adjusting means along saidscrew threads, whereby to cause said secondary member to move toward oraway from said first stationary member.
 9. The clamping mechanism ofclaim 8 wherein said moving means comprises an endless chain whichpasses around sprocket members carried by said adjusting means to moveeach of said adjusting means an equal distance, and means for movingsaid endless chain.
 10. The clamping mechanism of claim 9 includingmeans for locking said moving means in any desired position.